Field of the Invention
The present invention relates to an intermediate fire-resistant film and to its use in an article of manufacture such as a laminated material, particularly in a laminated glass.
Laminated glasses are ordinarily used in buildings and in means of transport as glazings having "safety protection" characteristics, allowing one to reduce the likelihood of disasters due to impact. The simplest and most well known of these laminated glasses consist of two or more sheets of glass stuck together by one or more intermediate films of plasticized polyvinylbutyral (PVB). While such structures are effective against impacts, they are insufficient as regards safety protection in case of fire. While shattering or collapse of the glass is always considered first in this type of situation, the actual foremost problem with these laminated glasses comes from the intermediate film of plasticized PVB, which becomes soft and melts much more rapidly than the glass, and which thereby, when there is a shattering of the glass, very soon ceases to maintain the pieces of glass by adhesion. The melted plasticized PVB then flows outside the glazing before catching fire with the extensive release of heat and smoke, well before the beginning of softening of the glass. The rapid softening of the plasticized PVB, the flow of the burning PVB, the release of smoke, and ignition of the plasticized PVB constitute dangers that must be remedied in this type of glazing before all else.
It is appropriate to note, in fact, that the majority of currently existing laminated glasses capable of resisting fire do not have the simple and economical structure mentioned above and are consequently free of the problems connected with this structure. Depending on the case, these fireproofing laminated glasses use intermediate films other than plasticized PVB or use additional intermediate elements, with these other films or elements having and/or giving the glass the desired fire resistance properties.
Among known fire-resistant laminated glasses containing intermediate films other than plasticized PVB are laminated glasses containing two or more sheets of glass assembled together by an intermediary gel or silicone film. French Patent No. 2,346,548 describes a fireproofing laminated glass in which the intermediate film is a gel containing a liquid with a high heat of vaporization, such as water, and a substance capable of forming an insulating foam under the effect of thermal radiation. This glass, designed so as to meet the largest number of fire resistance criteria and consequently classified in the "insulating fire-retarding elements," nevertheless has a lower resistance to freezing and to impacts than a traditional laminated glass using an intermediate film of plasticized PVB; its thickness is also greater.
Another example of a glass with particularly good fireproofing properties is described in French Patent No. 2,394,394 describing a laminated glass containing two sheets of glass between which is an intermediate film of a silicone elastomer. The major disadvantage of this product is its extremely high price in comparison with the price of a traditional laminated glass using an intermediate film of plasticized PVB.
Several laminated glasses using intermediate films of PVB as well as additional intermediate elements that have and/or give said glasses fire resistance properties are also available. U.S. Pat. No. 5,219,630 describes a glazing that has an intermediate film, which can be a PVB film, in which a network of metallic wires is inserted for the purpose of retarding the collapse of the intermediate film and the glazing. However, besides the fact that such a network is visible, a means of this type does not really slow down the melting of the intermediate material and does not prevent its final flow or ignition starting from a certain temperature. Laminated glasses in which layers of material functioning as a thermal insulator in case of fire are inserted between each surface of the intermediate PVB film and the adjacent sheets of glass are also described in patents such as U.S. Pat. No. 4,071,649. Such an intermediate film structure with three distinct layers is nevertheless more complex than a single intermediate film of plasticized PVB and also leads to thicker glazings. This structure also does not prevent the final flow of the PVB or its ignition, even if it does retard the completion of these two events.
A last example of a currently existing laminated glass capable of resisting fire, which cannot really be classified in one or the other of the two aforementioned categories (intermediate film other than PVB--additional intermediate elements), is given in the European Patent No. 227,633. This patent proposes mixing, with the PVB used to produce the intermediate film of a laminated glass, a nonnegligible quantity of a particular plasticizing mixture composed of various additives with different functions, so as to obtain an intermediate film of plasticized PVB having a modified composition capable of reacting, in case of fire, in order to resist the fire. The mixture of additives added to the PVB in this patent is complex, however, and obliges one to produce a particular plasticized PVB for the production of a laminated glass with resistance to fire.
In a general way, it is appropriate to note that the existing laminated glasses designed for resisting fire have more complex structures and/or are more costly (because they require in particular the development of special intermediate films with a specific composition or containing several layers) than the traditional laminated glasses using ordinary intermediate films of plasticized PVB. Furthermore, their impact resistance properties are often worse than those of said traditional laminated glasses.